David Carrasco

Toxicity of Aphanizomenon ovalisporum (Cyanobacteria) in a Spanish water reservoir

The summer phytoplankton population in the Arcos reservoir (Spain), an oligotrophic/mesotrophic waterbody, was dominated by the cyanobacterium Aphanizomenon ovalisporum, accompanied by Planktothrix cf. agardhii and smaller proportions of other cyanobacterial species. The bloom remained for approximately 1 month in the summer of 2004. The Arcos reservoir had moderate conductivity, up to 1756 µS cm−1, and relatively high nutrient concentrations. Planktonic biomass collected during the bloom period was found to be toxic by the Artemia salina bioassay. Analytical procedures based on high-performance liquid chromatography (HPLC) indicated the absence of microcystins and anatoxin-a but clearly demonstrated the presence of cylindrospermopsin (CYN), suggesting that CYN from Aphanizomenon ovalisporum was the compound responsible for the toxicity in the bioassay. Measurements indicate that up to 9.4 µg CYN l−1 were present in the sestonic fraction. This is the first report of high concentrations of this cyanotoxin in European waters and the first known instance of a massive bloom of the cyanobacterium Aphanizomenon ovalisporum in Spain.

Natural Photodegradation of the Cyanobacterial Toxins Microcystin and Cylindrospermopsin

Microcystins (MC) and cylindrospermopsin (CYN) are potent toxins produced by diverse cyanobacterial genera found in waterbodies throughout the world. In the present study, and in order to achieve a better understanding of the fate of cyanobacterial toxins in the environment, we assessed the photodegradation of MC and CYN along the water column and by different radiation bands of the natural solar spectrum: photosynthetic active radiation (PAR), UV-A, and UV-B. Photodegradation of CYN seemed to be highly dependent on UV-A and was very low under natural conditions. This fact could be one of the reasons explaining the high extracellular CYN concentration found in diverse waterbodies. Microcystin photodegradation was higher, all three radiation bands (PAR, UV-A, and UV-B) being responsible for its degradation, although PAR and UV-A were more efficient because of their high natural irradiance. Modeling of MC photodegradation along the watercolumn was performed, using specific MC breakdown rates for the different radiation bands and including calculated attenuation coefficients for these bands. As a result, we suggest that rapid and efficient MC photodegradation may be expected in shallow systems or thin mixed layers.

Cyanobacterial abundance and microcystin occurrence in Mediterranean water reservoirs in Central Spain: microcystins in the Madrid area

The occurrence and concentration of microcystins were analysed in sestonic samples from seven water reservoirs in the Madrid region (Spain) between July and November in two consecutive years (2002 and 2003). The data collected indicate that microcystins were present on several occasions in both years in all the reservoirs studied. The months of maximum risk for cyanotoxin occurrence were July, September and October, when Microcystis typically dominated the phytoplankton community. Four of the seven reservoirs exhibited conspicuous blooms, three of which were toxic. Of the samples in 2002 and 2003, 45% and 70%, respectively, contained microcystins (Mc). In four of the reservoirs, microcystin concentrations were higher than the WHO recommended limit for drinking water (1 µg Mc l−1) and, in one of the reservoirs (Santillana), its concentration, was extremely high (up to 70 µg Mc l−1). Concentrations above 1 µg l−1 were found during several months in this reservoir. Mc/chl a ratios in net-concentrated samples ranged from 0.0002 to 2.24 μg Mc (μg chl a)−1 with a mean value of 0.31 µg Mc (μg chl a)−1. From the data obtained we can conclude that M. aeruginosa is the main producer of microcystins in freshwaters from the Madrid region.

Anatoxin-a occurrence and potential cyanobacterial anatoxin-a producers in Spanish reservoirs1

The presence of anatoxin‐a (ANA) in Mediterranean region freshwaters has been reported only recently, and this work presents the first survey for ANA on a national scale in such waters. Fourteen reservoirs were sampled over a distance of 550 km from northeast to southwest Spain. Genera of cyanobacteria with ANA‐producing members in other countries were present in all of the Spanish reservoirs, but this toxin was detected in only one reservoir. The maximum ANA concentration detected was 0.31 μg · L−1, and the most probable producer was Anabaena flos‐aquae (Lyngb.) Bréb. ex Bornet et Flahault. These findings suggest that in spite of the abundant cyanobacterial populations, ANA may be of lower occurrence in the Spanish reservoirs investigated than in other European freshwaters, and different strains of potential ANA producers occur in different ecoregions.

Epiphytic Cyanobacteria on Chara vulgaris Are the Main Contributors to N 2 Fixation in Rice Fields

ABSTRACT The distribution of nitrogenase activity in the rice-soil system and the possible contribution of epiphytic cyanobacteria on rice plants and other macrophytes to this activity were studied in two locations in the rice fields of Valencia, Spain, in two consecutive crop seasons. The largest proportion of photodependent N2 fixation was associated with the macrophyte Chara vulgaris in both years and at both locations. The nitrogen fixation rate associated with Chara always represented more than 45% of the global nitrogenase activity measured in the rice field. The estimated average N2 fixation rate associated with Chara was 27.53 kg of N ha−1 crop−1. The mean estimated N2 fixation rates for the other parts of the system for all sampling periods were as follows: soil, 4.07 kg of N ha−1 crop−1; submerged parts of rice plants, 3.93 kg of N ha−1 crop−1; and roots, 0.28 kg of N ha−1 crop−1. Micrographic studies revealed the presence of epiphytic cyanobacteria on the surface of Chara. Three-dimensional reconstructions by confocal scanning laser microscopy revealed no cyanobacterial cells inside the Chara structures. Quantification of epiphytic cyanobacteria by image analysis revealed that cyanobacteria were more abundant in nodes than in internodes (on average, cyanobacteria covered 8.4% ± 4.4% and 6.2% ± 5.0% of the surface area in the nodes and internodes, respectively). Epiphytic cyanobacteria were also quantified by using a fluorometer. This made it possible to discriminate which algal groups were the source of chlorophyll a. Chlorophyll a measurements confirmed that cyanobacteria were more abundant in nodes than in internodes (on average, the chlorophyll a concentrations were 17.2 ± 28.0 and 4.0 ± 3.8 μg mg [dry weight] of Chara−1 in the nodes and internodes, respectively). These results indicate that this macrophyte, which is usually considered a weed in the context of rice cultivation, may help maintain soil N fertility in the rice field ecosystem.

Incorporation of Different N Sources and Light Response Curves of Nitrogenase and Photosynthesis by Cyanobacterial Blooms from Rice Fields

In this work, we estimate the contributions of the different sources of N incorporated by two N2-fixing cyanobacterial blooms (Anabaena sp. and Microchaete sp.) in the rice fields of Valencia (Spain) during the crop cycles of 1999 and 2000, and evaluate the response of nitrogenase and C assimilation activities to changing irradiances. Our results show that, far from the generally assumed idea that the largest part of the N incorporated by N2-fixing cyanobacterial blooms in rice fields comes from N2 fixation, both cyanobacterial blooms incorporated about three times more N from dissolved combined compounds than from N2 fixation (only about 33–41% of the N incorporated came from N2 fixation). Our results on the photodependence of C and N2 fixation indicate that in both cyanobacterial blooms, N2 fixation showed a steeper initial slope (α) and was saturated with less irradiance than C fixation, suggesting that N2 fixation was more efficient than photosynthesis under conditions of light limitation. At saturating light, N2 fixation and C fixation differed depending on the bloom and on the environmental conditions created by rice plant growth. Carbon assimilation but not nitrogenase activity appeared photoinhibited in the Anabaena but not in the Microchaete bloom in August 1999, when the plants were tall and the canopy was important, and there was no limitation of dissolved inorganic carbon. The opposite was found in the Microchaete bloom of June 2000, when plants were small and produced little shade, and dissolved inorganic carbon was very low.

Sedimentation Patterns of Toxin-Producing Microcystis Morphospecies in Freshwater Reservoirs

Understanding the annual cycle of Microcystis is essential for managing the blooms of this toxic cyanobacterium. The current work investigated the sedimentation of microcystin-producing Microcystis spp. in three reservoirs from Central Spain during the summer and autumn of 2006 and 2007. We confirmed remarkable settling fluxes during and after blooms ranging 106–109 cells m−2 d−1, which might represent 0.1%–7.6% of the organic matter settled. A comprehensive analysis of the Valmayor reservoir showed average Microcystis settling rates (0.04 d−1) and velocities (0.7 m d−1) that resembled toxin settling in the same reservoir and were above most reported elsewhere. M. aeruginosa settling rate was significantly higher than that of M. novacekii and M. flos-aquae. Despite the fact that colony sizes did not differ significantly in their average settling rates, we observed extremely high and low rates in large colonies (>5000 cells) and a greater influence of a drop in temperature on small colonies (<1000 cells). We found a 4–14 fold decrease in microcystin cell quota in settling Microcystis of the Cogotas and Valmayor reservoirs compared with pelagic populations, and the hypothetical causes of this are discussed. Our study provides novel data on Microcystis settling patterns in Mediterranean Europe and highlights the need for including morphological, chemotypical and physiological criteria to address the sedimentation of complex Microcystis populations.

Development of cyanobacterial blooms in Valencian rice fields

Seasonal variation of biomass, photosynthetic pigments and C and N contents of the cyanobacterial blooms developed during the crop cycles of 1998, 1999 and 2000 at two locations in Valencian rice fields (Spain) were studied to find their potential contribution to soil fertility. Blooms of Gloeotrichia sp., Gloeocapsa sp., Microchaete sp., and Nostoc sp. were small and dispersed and appeared only for a few weeks in the experimental fields of the Tancat de Malta location during the crop season of 1998. The biomass of these blooms ranged from 4.4 to 12.8 mg dry wt cm−2. A bloom of Anabaena sp. was found in the same location during most of the crop cycles of 1998 and 1999, covering up to 80% of the experimental fields. The biomass of this bloom ranged from 2.7 to 11.4 mg dry wt cm−2. In the moment of its maximal extension (July 1998) we estimate a total biomass of 2,100 kg dry wt for this bloom, equivalent to 420 kg dry wt ha−1. In the Sueca location a bloom of Microchaete sp. was observed in August 1999 and during most of the crop cycle of 2000, when it covered a wide area (60–70%) of the experimental field. The biomass of this bloom ranged from 0.9 to 5.6 mg dry wt cm−2. Carbon contents of the blooms ranged from 150 to 310 μg mg−1 dry wt and N contents ranged from 11 to 30.2 μg mg−1 dry wt. Taking into account their N content and their extension we estimate that the incorporation of cyanobacterial blooms could add 4–12 kg N ha−1 to soil. From an agronomical point of view this means that, in spite of their impressive aspect, cyanobacterial blooms can only fulfill a small proportion of the N requirements of rice plants.